A biosensor may be denoted as a device which may be used for the detection of an analyte that combines a biological component with a physicochemical or physical detector component.
For instance, a biosensor may be based on the phenomenon that capture molecules immobilized on a surface of a biosensor may selectively hybridize with target molecules in a fluidic sample, for instance when an antibody-binding fragment of an antibody or the sequence of a DNA single strand as a capture molecule fits to a corresponding sequence or structure of a target molecule. When such hybridization or sensor events occur at the sensor surface, this may change the electrical properties of the surface and the volume directly above the surface which can be detected as the sensor event.
Many suitable specific binding pair candidates are known per se, which are typically based on a lock-and-key type interaction between a receptor molecule and a molecule, e.g. a drug. This makes a sensing apparatus such as an assay-based apparatus particularly suitable to determine the presence or absence of specific proteins and other biological compounds such as DNA, RNA, hormones, metabolites, drugs and so on, or to determine the activity and function of active and catalytic biomolecules such as proteins, peptides, prions, enzymes, aptamers, ribozymes and deoxyribozymes. For instance, immunoassays are already used to determine the specific amount of specific proteins in body fluids to aid further diagnosis and treatment.
Due to advances in semiconductor technology, it has become feasible to detect single capture events on a sensing surface of such sensors. An example of such a sensor is disclosed in PCT patent application WO 2009/047703, in which a capture molecule forms an insulating layer of a capacitor, with the plates or sensing electrodes of the capacitor formed by a conductive sensing surface and a fluid sample respectively. A capture event causes a change in the dielectric constant of the insulating layer including the volume directly above the sensor surface in which a capture event takes place, which affects the capacity of the capacitor. The change in capacitance can be measured, e.g. as a bias on a current through a transistor, as is the case in this application.
An alternative arrangement is disclosed in PCT patent application WO 2008/132656, in which an extended gate field effect transistor is disclosed with capture molecules on the surface of the extended gate as the sensing electrode, such that the gate potential of the transistor can be altered by capture events.
Another type of biosensor that has been gaining considerable attention is an assay-type biosensor in which antibodies are bound to magnetic beads, which are attracted to a sensing surface carrying further antibodies by a magnetic force, with the analyte of interest binding the magnetic beads to the sensing surface by forming a binding pair with the antibodies and the further antibodies. Examples of such assays are for instance given in PCT patent application WO 2007/060601, although the biosensors based on such magnetic beads are less suitable for detection at single binding event resolution. Instead, a bulk magnetic signal is detected which has a magnitude that scales with the concentration of the analyte of interest in the investigated sample.
As is for instance disclosed in WO 2008/132656 and WO 2009/047703, it is possible to functionalize different parts of the array of biosensors with different receptor molecules such that different analytes can be detected in parallel. This is of course attractive, as it reduces the overall detection times for different analytes. However, the diversification of the functionalization of the different biosensors on the IC is not trivial due to the small sensing area of the most advanced CMOS biosensors. In order to be able to detect a specific binding event, it must be known which biosensor elements, e.g. electrode or electrodes, contains which receptor molecule. This therefore requires careful placement of the receptor molecules on the electrodes, which may be achieved by spotting techniques for instance, in which a droplet or spot of a solution that contains a specific receptor is placed on a selected part of the biosensor surface.
However, modern manufacturing techniques facilitate the provision of ICs that comprise biosensor arrays, e.g. arrays of electrodes that have a pitch substantially smaller than the size of such droplets, such that standard spotting techniques are unsuitable for providing individual electrodes with separate receptors. For instance, it is routinely possible to provide an area of a few hundred microns squared whereas the diameter of a single spot as provided by common spotting techniques is typically in the region of 100-300 microns, i.e. several orders too large to address individual electrodes which sizes are in the 100 nm range. These kinds of spot sizes are in the same order of magnitude than the full sensor array which would not allow to deposit different spots on one biosensor. Smaller spots may be provided e.g. by using atomic force microscope (AFM)-like tip spotting techniques such as the commercial systems of BioForce or Nanodrop, but the cost of such techniques is prohibitive for large scale industrial application. Moreover, such sensor functionalization typically takes place in a biological laboratory, where such advanced spotting technologies may not be available.
Apart from the above functionalization problems, a further problem associated with such biosensor arrays is that the single molecule detection signal typically is rather weak, such that long acquisition times may be required to achieve an acceptable signal to noise ratio.